Nickel is one of the most important electrodeposited metals due to its excellent decorative and corrosion-resistance properties. Most nickel electrolyte systems are based on the Watts plating bath, which generally contains nickel sulfate, nickel chloride, and boric acid. Electroplating baths have been developed to produce ductile, low-stress, high leveling, semi-bright and bright nickel deposits. A multilayered nickel deposit, containing a sulfur-free semi-bright nickel layer with a sulfur-containing bright nickel layer, significantly improves corrosion protection.
A variety of organic compounds are used as brighteners in nickel baths to provide a bright, level, and ductile nickel deposit. Nickel brighteners are generally divided into two classes. Class I, or primary, nickel brighteners include compounds such as aromatic or unsaturated aliphatic sulfonic acids, sulfonamides, sulfonimides, and sulfimides. Class I brighteners are used in relatively high concentrations and produce a hazy or cloudy deposit on the metal substrate. Class I brighteners decompose during the electroplating process, and sulfur is incorporated into the deposit, which reduces the internal tensile stress of the deposit.
Class II, or secondary, nickel brighteners are used in combination with Class I brighteners to produce a fully bright and leveled nickel deposit. Class II brighteners are generally unsaturated organic compounds. A variety of organic compounds containing unsaturated functional groups such as aldehydic, olefinic, acetylinic, nitrite, and pyridine groups have been used as Class II brighteners. Typically, Class II brighteners are derived from acetylinic or ethylenic alcohols, ethoxylated acetylenic alcohols, coumarins and pyridine based compounds. Mixtures of such unsaturated compounds with mixtures of Class I brighteners are combined to obtain maximum brightness or ductility for a given rate of leveling. Class II brighteners, however, cause brittleness and increase internal stress in a deposit, and, therefore, cannot be used alone.
A variety of amine compounds have been reported as brightening or leveling agents. Several patents describe the use of acyclic amines as Class II brighteners that require the use of other Class I and Class II brighteners. U.S. Pat. No. 4,077,855 is directed to the use of olefinic or acetylenic sulfobetaines and carboxybetaines. U.S. Pat. Nos. 4,054,495 and 4,435,254 disclose the use of acetylenic amines in combination with acetylenic compounds as an effective brightener and leveling system. U.S. Pat. No. 5,840,986 describes the synthesis of N-alkyl-substituted aminoalkynes, which could potentially be used as a brightener in nickel baths.
Other patents disclose the use of single or multi ring nitrogen containing heterocyclic amines as Class II brighteners for bright nickel electroplating systems. U.S. Pat. Nos. 2,876,177 and 3,862,019 discuss the use of pyridine sulfobetaines. U.S. Pat. No. 4,212,709 expands on the pyridine-based brighteners to include mononuclear and polynuclear aromatic heterocyclic nitrogen bases. U.S. Pat. Nos. 5,438,140 and 5,45,727 describe the use of alkyl derivatives of nitrogen containing heterocycles as brighteners for nickel electroplating systems. U.S. Pat. Nos. 5,606,067 and 5,611,906 disclose the preparation of these compounds.
The above mentioned brightener systems, however, may exhibit one or more of the following limitations: poor ductility; limited low-current density coverage; poor leveling at low-current densities; burning at high current densities; or decreased receptivity to subsequent chromium deposits. It is therefore desirable to develop an brightener system for an acidic nickel plating bath that will produce an extremely ductile, leveled and highly bright deposit over a wide range of current densities.